The present invention relates to scan line non-linearity in a Raster Output Scanning (ROS) system and, more particularly, to an aspheric output window or aspheric mirror in the ROS system to correct the scan line non-linearity.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. This laser beam is modulated in conformance with the image information. The modulated beam is transmitted through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet of the rotating polygon mirror and thereafter is focused to a xe2x80x9cspotxe2x80x9d on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements (xe2x80x9cpixelsxe2x80x9d) of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper.
The raster output scanner has various optical components of mirrors and lenses to collimate, expand, focus and align the modulated scanning beam. These optical components are fixedly mounted within a housing frame, which is positioned within a printer machine frame, so that the modulated and shaped scanning beam emerging from an output window in the housing is directed in a scan line which is perpendicular to the photoreceptor surface. The lines will be formed in parallel across the surface of the photoreceptor belt.
One problem inherent in the optical system of a ROS is xe2x80x9cscan linearityxe2x80x9d. Scan linearity is the measure of how equally spaced the spots are written in the scan direction across the entire scanline. Typical scan linearity curves start at zero position error at one end of a scan having a positive lobe of position error across the scanline, cross the center of scan with zero position error and then have a negative lobe of position error across the remainder of the scanline toward the other end of the scan. Scan linearity curves may have image placement errors of zero at several locations across the scanline. Ideally, the curve would be at zero across the entire scanline.
The main function of the f-theta lens group between the rotating polygon mirror and the photoreceptor is to control the scan linearity, in terms of uniform spot displacement per unit angle of polygon rotation.
Bow distortions occur for a scanning beam in a ROS from the unavoidable imprecisions in the manufacture and mounting of the lenses and mirrors of the optical elements of the ROS. Scanline bow arises from the very nature of optical scanning systems, where the beam is offset in the cross-scan direction from the ideal horizontal straight line in the scan direction of the scan line on the recording medium. The scanline bow occurs because the magnification of the optical system of the ROS varies across the cross-scan direction as the beam propagates through the optical system.
Depending upon the accumulation of optical tolerances, the bow may bend in the middle of the scan line about a central mid-point in either cross-scan direction. A bow where the central mid-point is higher than the rest of the scan line is called a xe2x80x9cfrownxe2x80x9d while a bow where the central mid-point is lower than the rest of the scan line is called a xe2x80x9csmilexe2x80x9d.
Scanline bow arises in both single and multiple beam ROS""s and is the overall curvature of the scan line relative to the ideal of a straight scan line.
In a multiple beam ROS system, each light source emitting a beam has its own bow curve. It is the maximum difference in the bow curves between the multiple light sources in a given system that defines the xe2x80x9cdifferential bowxe2x80x9d.
A plurality of ROS units can be used in a color xerographic ROS printer. Each ROS forms a scan line for a separate color image on a common photoreceptor belt. Each color image is developed in overlying registration with the other color images from the other ROS units to form a composite color image which is transferred to an output sheet. Registration of each scan line of the plurality of ROS units requires each image to be registered to within a 0.1 mm circle or within a tolerance of xc2x10.05 mm.
One solution to error in registration of the scan line is to rotate the output window of the ROS, as taught in U.S. Pat. No. 5,821,971 and U.S. Pat. No. 5,889,545, both commonly assigned as the present application and both hereby incorporated by reference. The transmissive output window has no optical power. However, rotation of the output window only rotates the beam uniformally along the entire scan line.
Another solution to scan line registration error is to twist the output window in the ROS system to create the required deflection of the projected scan line, as taught in pending U.S. patent application Ser. No. 09/219,004, commonly assigned as the present application and hereby incorporated by reference. Once again the transmissive output window has no optical power.
It is an object of the present invention to correct non-linearity of the scan line in a ROS by using an aspheric output window or aspheric mirror.
It is another object of the present invention to correct bow of the scan line and non-linearity of the scan line in a ROS by using an aspheric output window or aspheric mirror.
According to the present invention, an aspheric optical element corrects the non-linearity of the scan line in a ROS. The optical element can be either the wobble correction mirror, the last optical element in the ROS, or the output window, subsequent to the ROS. The optical element deflects the scan beam to cancel the non-linearity of the scan line caused by the residual errors in the ROS lens design. The aspheric optical element can also correct scan line bow.